The rear brake light is activated in two ways, one from the lever on the handle bar on the right side of the bike (front brake lever) and the other is from the foot brake on the right side of the bike (rear brake pedal).
The easiest one to check first would be the rear brake. Turn the key on so your lights are on. At the rear brake pedal, find a long thin rod that goes vertical up the bike. It will be connected to a spring, which is connected to a black kind of round thing. Off of the round black thing will be two wires. Unplug them and see if the light goes out.
The front one is easy to do, just a little more difficult to reach. Follow the wire coming from your front brake perch. It will go behind the headlight. You may have to stoop down or get on your knees to get to it. Anyways, follow the wire to the connector and unplug it. If this turns off your brake light then you know its the front.
Here is a picture of the rear brake pedal connector:
And the front. Mine is a 1990 so it will be different from yours, but I assume it will be similar.
At the lever:
Follow it down the handlebar to behind the headlight:
And just unplug it:
There could be other things wrong as well, but this should narrow it down.